1. Field of the Invention
The present invention relates to a nucleic acid encoding the enzyme hyaluronate synthase, and to the use of this nucleic acid in the preparation of recombinant cells for the production of the hyaluronate synthase enzyme and hyaluronic acid. Hyaluronate is also known as hyaluronic acid or hyaluronan.
2. Description of the Related Art
The incidence of streptococcal infections is a major health and economic problem worldwide, particularly in developing countries (Rotta, 1988). One reason for this is due to the ability of Streptococcal bacteria to grow undetected by the body's phagocytic cells (i.e., macrophages and polymorphonuclear cells (PMNs). These cells are responsible for recognizing and engulfing foreign microorganisms. One effective way the bacteria evade surveillance is by coating themselves with polysaccharide capsules, such as hyaluronic acid (HA) capsules. (Kendall et al., 1937). Since HA is generally nonimmunogenic (Quinn and Singh, 1957), the encapsulated bacteria do not elicit an immune response and are, therefore, not targeted for destruction. Moreover, the capsule exerts an antiphagocytic effect on PMNs in vitro (Hirsch, et al., 1960) and prevents attachment of Streptococcus to macrophages (Whitnack, et al., 1981). Precisely because of this, in group A and group C Streptococci, the HA capsules are major virulence factors in natural and experimental infections (Kass, et al., 1944; Wessels, et al., 1991). Group A Streptococcus are responsible for numerous human diseases including pharyngitis, impetigo, deep tissue infections, rheumatic fever and a toxic shock-like syndrome (Schaechter, et al., 1989). The group C Streptococcus equisimilis is responsible for osteomyelitis (Earson, 1986), pharyngitis (Benjamin, et al., 1976), brain abscesses (Dinn, 1971), and pneumonia (Rizkallah, et al., 1988; Siefkin, et al., 1983).
Structurally, HA is a high molecular weight linear polysaccharide of repeating disaccharide units consisting of N-acetylglucosamine (GlcNAc) and glucuronic acid (GlcA) (Laurent and Fraser, 1992). HA is the only glycosaminoglycan synthesized by both mammalian and bacterial cells particularly groups A and C Streptococci. Some Streptococcus strains make HA which is secreted into the medium as well as HA capsules. The mechanism by which these bacteria synthesize HA is of interest since the production of the HA capsule is a very efficient and clever way that Streptococci use to evade surveillance by the immune system.
HA is synthesized by both mammalian and Streptococcus cells by the enzyme hyaluronate synthase, that has been localized to the plasma membrane of Streptococcus (Markovitz, et al., 1962). The synthesis of HA in these organisms is a multi-step process. Initiation involves binding of an initial precursor, UDP-GlcNAc or UDP-GlcA. This is followed by elongation which involves alternate addition of the two sugars to the growing oligosaccharide chain. The growing polymer is extruded across the bacterial plasma membrane region of the cell wall and into the extracellular space. Although the HA biosynthetic system was one of the first membrane heteropolysaccharide synthetic pathways studied, the mechanism of HA synthesis is still not understood. This may be because in vitro systems developed to date are inadequate in that de novo biosynthesis of HA has not been accomplished. Chain elongation but not new chain initiation occurs.
The direction of HA polymer growth is a matter of disagreement. Addition of the monosaccharides could be to the reducing (Prehm, 1983) or nonreducing (Stoolrniller, et al., 1969) end of the growing HA chain. In addition, other questions that need to be addressed are (i) whether nascent chains are linked covalently to a protein, to UDP or to a lipid intermediate, (ii) whether chains are initiated using a primer, and (iii) the mechanism by which the mature polymer is extruded through the plasma membrane of the Streptococcus. Understanding the mechanism of HA biosynthesis may allow development of alternative strategies to control Streptococcal. infections by interfering in the process.
Group C S. equisimilis strain D181 synthesizes, and secretes HA. Investigators have used this strain and group A strains, such as A111, to study the biosynthesis of HA and to characterize the HA-synthesizing activity in terms of its divalent cation requirement (Stoolmiller, et al., 1969), precursor (UDP-GlcNAc and UDP-G1cUA) utilization (Ishimoto, et al., 1967; Markovitz, et al., 1959) and optimum pH (Stoolmillér, et al., 1969). The HA synthase enzyme has been studied for approximately 30 years, but has not yet been identified or purified. Although a 52-kD protein has been putatively suggested as the HA synthase (Prehm, et al., 1986), this report is now known to be in error. Furthermore, no one has successfully purified to homogeneity an active enzyme. Moreover, it's not clear whether a bona fide HA synthase molecule is all that is needed for the generation of hyaluronic acid, or whether it might act in concert with other cellular components or subunits. Thus, totally ex vivo methods of producing HA have not been forthcoming.
Typically, HA has been prepared commercially by isolation from either rooster combs or extracellular media from Streptococcal cultures. One method which has been developed for preparing HA is through the use of cultures of HA-producing streptococcal bacteria. U.S. Pat. No. 4,517,295, describes such a procedure, wherein HA-producing Streptococci are fermented under anaerobic conditions in a CO2-enriched growth medium. Under these conditions, HA is produced and can be extracted from the broth. It is generally felt that isolation of HA from rooster comb is laborious and difficult, since one starts with HA in a less pure state. The advantage of isolation from rooster comb is that the HA produced is of higher molecular weight. However, preparation of HA by bacterial fermentation is easier, since the HA is of higher purity to start with. Usually, however, the molecular weight of HA produced in this way is smaller than that from rooster combs. Therefore, a technique that would allow the production of high molecular weight HA by bacterial fermentation would be an improvement over existing procedures.
High molecular weight HA has a wide variety of useful applications—ranging from cosmetics to eye surgery (Laurent and Fraser, 1992). Due to its potential for high viscosity and its high biocornpatibility, HA finds particular application in eye surgery as a replacement for vitreous fluid. HA has also been used to treat racehorses for traumatic arthritis by intra-articular injections of HA, in shaving cream as a lubricant, and in a variety of cosmetic products due to its physiochemical properties of high viscosity and its ability to retain moisture for long periods of time. In general, the higher molecular weight the HA that is employed the better. This is because HA solution viscosity increases with the average molecular weight of the individual HA polymer molecules in the solution. Unfortunately, very high molecular weight HA, such as that ranging up to 107, has been difficult to obtain by currently available isolation procedures.
To address these or other difficulties, there is a need for new methods and constructs that can be used to produce HA having one or more improved properties such as greater purity or ease of preparation. In particular, there is a need to develop methodology for the production of larger amounts of relatively higher molecular weight and purity HA than is available from current technology. The present invention addresses one or more shortcomings in the art through the application of recombinant DNA technology.